The present invention concerns a printing apparatus capable of performing effective printing by two types of printing methods for printing from a small number of sheets to a large number of sheets. Especially, the present invention relates to the printing apparatus being capable of obtaining printed matters of clearness and intense density, and a recording method for use in such apparatus.
In the field of a digital printing apparatus of high speed printing at a low running cost, stencil printing has been popularized. In such stencil printing, a thermoplastic resin film of a stencil sheet is melted and perforated by a heating means such as a thermal head which generates heat in dot-like form as character and picture information according to an electrical signal.
In using the digital printing apparatus, owing to irregularity of contact-pressure between the thermal head and a platen roller, there may occur perforation failure, creasing in the stencil sheet and conveyance failure of the stencil sheet.
Further, although an existent digital printing apparatus is useful in the case of printing a large number of identical printed matter, if the number of sheets for printing is small, the printing cost is rather increased since the stencil sheet is used. With regard to the background thus stated, if the digital printing apparatus includes heatsensitive recording paper or heatsensitive transfer recording paper for a small number of sheet printing, it may be considered possible that one printing method is optionally selected from among the stencil printing and printing by the heatsensitive recording paper according to the number of printing sheets. However, such constitution enlarges the digital printing apparatus and further requires at least two types of recording paper, ie normal printing paper and the heatsensitive recording paper, which causes a drawback of complicating the operation of the apparatus.
Further, a composite type printing apparatus has also been proposed which combines different printing methods of using common printing paper. The composite type printing apparatus conducts electrophotographic printing in the case where a number of printing sheets is small, and also conducts printing by using heat sensitive stencil sheet in the case where a number of printing sheets is large. But it has a drawback that the entire system is complicated, expensive and enlarged in the size.
On the other hand, in order to obtain colored printed matters by the digital printing apparatus, an ink-charged drum has to be prepared for each of colors. Such apparatus requires, even in a partial color printing, a troublesome operation of exchanging the drum at every time when previous printing in different color is finished. This operation worsens the efficiency.
Further, the applicant examined perforation condition in a stencil sheet processed by the digital printing apparatus. As a result, it is discovered that an excessive amount of ink passed through an area of a heatsensitive stencil sheet where dot-shape holes were perforated in a successive manner by the thermal heat. This deteriorated printing quality by causing bleeding of an image like a character formed on printing paper. Further, set-off and seeping-through in printed matters were apt to occur.
Further, it is also discovered that an adequate amount of ink had properly passed through an area in a stencil sheet where dot-shape holes perforated by the thermal head were independent and not connected with each other. Consequently, an image like a character formed on printing paper was clear, set-off and seeping-through were not found in printed matters, thereby obtaining printed matters of high quality.
An object of the present invention is to provide a recording apparatus and a recording method capable of performing an effective printing from a small number of sheet to a large number of sheet at a low running cost by selectively using plural types of printing method, thereby obtaining printed matters of high quality with clearness and intense density.